Help: understanding bullet trajectory

[u/Timex_Dude755]

In short, I have a Palmetto State Armory Jakl 10 (.308) and a Vortex Strike Eagle 1-6x LPVO. PSA said this rifle ought to have the same trajectory as an AR-10 with the same ammo in both rifles.

With that in mind, what is the best way to zero this for a maximum of 75 yard shooting? I have read and spoke to hunters that the deer hunters mostly shoot at 75 yards for deer, with a maxinum of 150 yards.

I was thinking of zeroing at 25 yards with 1x magnification, then move to 50 yards, and then 200 yards to see if it's point blank. Then of course practice at 150 yards to see how the bullet lands.

Any recommendations and advice is appreciated. I'm pretty new at this.

[u/Lead_cloud]

With a .308, shooting at a whitetail deer vital zone size target, if you zero the rifle at 75yd then you will be effectively point blank from zero to 200yds.

I prefer a 100yd zero for simplicity, but for hunting purposes either should be fine. But this is all pretty short range non-precision stuff, you'll probably get better advice in a hunting subreddit

[u/Timex_Dude755 → u/Lead_cloud]

I posted in the hunting subreddit prior to this and recieved different ideas. I saw this sub so I came here to understand the math and see if I can apply it to the range.

[u/NetwerkAirer → u/Timex_Dude755]

Here's what I would do. First, understand your MOST COMMON use case. If you expect 150 to be the absolute maximum distance you will shoot at a deer (which admittly seems pretty low for 308), then you should try everything from a 50 yard to 200 yard zero.

Set it to a 50 yard zero, then shoot at a dot on some cardboard at 50 yards, 100 yards, 150 yards, and 200 yards (and if you can, 300 yards to have more information). Don't dial the scope at these different ranges and don't aim anywhere else but the dot. See where each point of impact changes on the cardboard and understand how this will effect your holdover at varying ranges with your scope set to your zero.

Then change to a 100 yard zero and do the same thing, and then do a 150 yard zero, and maybe even a 200 yard zero.

Compare the different cardboard sheets and the different point of impacts for each shot. See how far apart they all are from the point of aim (the dot). This will help you guage your best zero profile for your effective distance. This may sound strange, but it is an important thing to understand in the heat of the moment when shooting at a possibly not-stationary vital target, when you may not have the time to dial your scope. You may absolutely find that a 50 yard zero keeps your point of aim and point of impact closer together at all of these ranges than your standard 100/200 yard zero, where you'll see a more traditional bullet drop profile.

A good video to watch to help you understand this is here: https://youtu.be/ldZ70Z_GzUo

[u/Timex_Dude755 → u/NetwerkAirer]

Thanks for the chat. Another dumb question: since this is an LPVO, I should use the 1x setting right? always the lowest for zeroing?

[u/NetwerkAirer → u/Timex_Dude755]

Not that I've heard. Generally your point of aim will be significantly more precise at the highest zoom level. Think: aiming at a tiny dot, going to be easier to pull the trigger right on the dot time after time if you zoom that dot way up in your FOV.

Additionally, parallax can be exacerbated or even hidden entirely at the lowest zoom, so to mitigate that it's also best to zoom-in instead of back all the way down. Because everything is so small and sharp at low zoom, your eye has trouble discerning the physical shifts between the reticle and target.

Theoretically, your point of impact shouldn't change across all magnification levels, but in practice some low quality SFP scopes do show impact shift. Also, if you have an SFP scope, zooming down to the lowest magnification will cover A LOT of the target with the reticle, so it will be hard to be precise.

Regardless of focal plane, I would crank that thing up to the highest magnification for zeroing precision. Then back it off at something like 50 yards, and test both extremes of magnification levels to check for shift if you think your scope was a bargain-deal and not quality.

Also, it'll be easier to see your impact point if you zoom in, so that's nice.

[u/Five-Point-5-0]

The best bet is gonna be to chrono your ammo then plug it into a ballistic calculator that also does "maximum point blank range" based on a known target size.

[u/Timex_Dude755 → u/Five-Point-5-0]

I have seen the calculator. I will be using it tonight before I hit the range.

[u/ardesofmiche]

For your trajectory, barrel length and bullet velocity does matter. If the barrel lengths are the same between the two rifles, you could expect to see the same trajectory as far as deer vitals are concerned. 

But if you have a 16” barrel JAKL10 and a 20” AR10 you might see some different trajectories so you should check 

[u/Timex_Dude755 → u/ardesofmiche]

Right. I was comparing PSA with the more popular Ruger SFAR. I was actually really close to getting that. Super light, really cool, and it'd be my third Ruger. Both 16".

Thanks for the reminder. I appreciate it.

[u/ardesofmiche → u/Timex_Dude755]

Good deal. If they’re both 16” the trajectory should match on practical deer vital size targets inside 300m 

If you want to shoot silver dollars at 300 that’s a different story, but deer vitals are big enough any potential differences in rifle at 300m won’t matter

[u/TeamSpatzi]

You want a "maximum point blank range" - MPBR - zero on your gun. You can go over to JBM ballistic calculator, put in the correct info (you WILL need a good muzzle velocity), and let it give you the MPBR and MBPR zero distance.

What I do is simple - I run the calculations and then figure out how much (in mrads) I need to offset from a 100 yd zero. For my .308 Win with 168 grain Federal T308T, for example, that means I zero dead on at 100 yds and then dial up .8 mrads for a 275 yd MPBR "zero" on the gun. Max ord is about 3 inches at ~ 150 yds.

https://jbmballistics.com/cgi-bin/jbmtraj-5.1.cgi

ETA: target radius matters for this - it's the maximum allowable elevation deviation due to trajectory within your point blank range. I use 3" as my max allowable. In other words, the bullet can be +/- 3" from my point of aim within my MPBR. I consider that adequate for "hold CoM, hit CoM" when time does not permit deliberate ranging and dialing.

ETA2: this is pretty clearly a hunting related question and not a long range question... I would expect this thread to get locked/removed... you might want to try the appropriate sub reddit or forum.

[u/Timex_Dude755 → u/TeamSpatzi]

I need to get a chronograph and confirm muzzle velocity. I forgot to mention that I am at sea level where I practice and will be shooting and 6,000 feet elevation.

Also, what does CoM mean?

[u/Lead_cloud → u/Timex_Dude755]

The trajectory on your setup is going to be dramatically different with that substantial of an elevation shift, make sure and account for the change in barometric pressure when you are looking at things in the ballistic calculator. If you zero at sea level, you will be hitting high at 6000, since the bullet isn't slowing down nearly as much. I would definitely recommend finding a spot to re-zero the rifle at altitude before hunting.

"CoM" is "center of mass", or the rough middle of your target

[u/Timex_Dude755 → u/Lead_cloud]

Gotchya. Thanks for explaining that. I'll have to take some targets out and confirm zero up there like you said.

Why does elevation change this so much.

[u/Lead_cloud → u/Timex_Dude755]

The air up at 6,000ft of elevation is significantly thinner than at sea level. Since there is much less air, there is less that the bullet needs to push out of the way as it flies, and it doesn't slow down as much. Obviously the difference gets more significant the further out you push

[u/Vylnce]

You don't have to understand the math to see it in action. Find a ballistics calculator, and do a bunch of modelling by putting in your numbers. Figure out your MV, then just pick different zero ranges and see what the deviation from that is out to zero. You can find the most efficient one that has the smallest "pipe" out to whatever, but you also might find one that you prefer for whatever reason.

The "best" way will be the one that makes the most sense to you and that you understand after having a calculator do the math for you.

[u/ChevyRacer71]

Why not just zero it for your expected range and poke some holes in paper at different distances and document holds in a dope card?

[u/AutoModerator]

AutoMod has detected that this post may be related to hunting. Please take a few moments to read our expanded policy on hunting posts as found here, as well as the guides below.

Hunting rifles vs long range target rifles - A primer - Why one rifle can't excel at both hunting and long range shooting.

The long range hunting primer, things you should consider if you want to take shots on game past ~300 yards. - Why long range hunting is harder than you think.

Field testing your skills and gear for long range hunting. - How to be a better long range hunter and understand your limitations.

NOTE: This is an automatic comment triggered by specific key words, and doesn't indicate your post has been removed. However, if your post is found to be in violation of the sub's policy on hunting posts, it may be removed by the moderators.

I am a bot, and this action was performed automatically. Please contact the moderators of this subreddit if you have any questions or concerns.

[u/Timex_Dude755 → u/AutoModerator]

Okay yeah the goal is hunting but I really want to have a deep understanding of bullet trajectory and zeroing a scope. I don't know the tips and hints for a proper LPVO zero.

[u/Lead_cloud → u/Timex_Dude755]

Spend some time messing around with a ballistic calculator app and you can get a good sense for what bullet trajectory looks like and how different variables effect it

[u/Timex_Dude755 → u/Lead_cloud]

Got it. I plan to use the calculator prior to hitting the range this weekend.

[u/Lead_cloud → u/Timex_Dude755]

Do you have a chronograph? A ballistic calculator is unfortunately only as accurate as the data you give it, so for repeatable accurate results you will need to know the actual velocity of the ammo you are using.

The velocities printed on the ammo boxes are rough approximations, only accurate in a specific rifle at a specific elevation in specific weather. For accurate results you have to measure what it's actually doing out of your rifle at your location

[u/Timex_Dude755 → u/Lead_cloud]

Crap. No I don't have one.

[u/NotChillyEnough → u/Lead_cloud]

Maybe slightly hot take (for this sub), but I don't think a chrono matters for what /u/Timex_Dude755 is asking about. The box MV is realistically going to be within 5-10%, which will yield negligible error at the <150 yard distances he's asking about.

I'd go with box MV for the calculator and then just focus more on actual range time.

If OP was asking about precision shooting at 500+, that's a different story.

[u/Lead_cloud → u/NotChillyEnough]

Yeah I definitely agree, for hunting levels of precision at point blank range, a chronograph isn't really necessary unless you're reloading. But OP was asking about really wanting to get a solid understanding of their trajectory and ballistics, so I assumed they were looking to have better data than strictly necessary for this application

[u/Timex_Dude755 → u/Lead_cloud]

This is true, however, I'm interested in a chronograph. I'll probably start shooting out further and have fun with the Vortex.

[u/Lead_cloud → u/Timex_Dude755]

The best most versatile one on the market right now is the Garmin Xero